This invention relates to apparatus and methods for the measurement of physical variables. More particularly, the invention concerns the measurement of such variables at a remote location.
Physical variables, such as pressure, temperature, atmospheric loading, etc., are required to be measured in many different applications in research and industrial procedures. Frequently, such variables must be measured at a remote location and information concerning the measured quantity must then be relayed to a receiving unit. In underwater communications systems utilizing sonar, for example, remote pressure sensors are employed to detect dynamic water pressure changes which contain information useful in communications or detection systems. Such sonar pressure sensors are commonly employed on sonobuoys and in towed sensor arrays.
In the past, sensors for use in sonar and other such remote applications have employed transducers which utilize the principles of magnetostriction or piezoelectricity to convert water pressure or other changes caused by a signal to changes in the amplitude of an electrical signal. When the information detected by a remote pressure sensor must be relayed to another location, such an amplitude modulated signal has commonly been converted in the prior art to a frequency modulated signal and then transmitted in a radio frequency broadcast to a receiver.
Such prior art methods of detecting and relaying information concerning pressure and other physical variables are subject to a number of disadvantages. Amplitude modulated sensors operate effectively only for relatively low frequency inputs. In addition, the output signal provided by such a sensor is at a very low level. As a consequence, the signals provided by known remote transducer designs tend to contain an excessive amount of electrical noise. Furthermore, relatively complex electronic circuitry is required in the design of such a transducer to provide the AM to FM conversion capability, with the result that prior art sensing systems have tended to be expensive, unreliable, and relatively short lived. In addition, prior art remote sensor designs have been subject to variations in output depending upon variations in the ambient temperature at the location of the sensor.
Consequently, a need has developed in the art for an improved sensing technique capable of sensing a physical variable at a remote location and relaying information concerning the variable. It would be advantageous for such a system to be highly reliable yet relatively inexpensive to construct. Furthermore, it would be desirable for such a system to provide a clear and distinct output signal with sufficient signal strength to be effectively transmitted to a distant receiver.